In the manufacture of semiconductor devices, acidic etching solutions are used for treating and etching wafers. Phosphoric acid (H3PO4) has been used for decades in immersion wet benches as an etching solution to etch silicon nitride films and other films. In a typical application, the phosphoric acid bath is maintained at a desired boiling condition by adding heat to the bath using in-line and/or tank wall heaters and balancing the water that is lost to boiling by adding liquid water back to the bath.
The phosphoric acid is typically delivered to the bath as a mixture with water, for example as an 85% phosphoric acid-15% water mixture (percentage by weight), which boils at a temperature of about 158° C. at atmospheric pressure. This is primarily done to reduce the viscosity of the phosphoric acid to a level that facilitates delivery of the etching solution in a manufacturing facility using conventional liquid chemical distribution systems. Heaters are used to bring this solution up to its boiling point. As the mixture boils, the hydration level falls as water vapor is driven off and the temperature (boiling point) rises until the desired processing temperature (typically a boiling point of 160° C. or 165° C.) is reached. Once at this boiling temperature, the heaters are typically driven at a constant power level to continue adding heat to the system and the temperature is controlled at the desired setpoint by regulating the addition of water. The wet bench processing tank in which wafers are immersed is open to the atmosphere, so this boiling of the solution occurs at atmospheric pressure. Because the pressure is fairly constant, by maintaining a constant temperature and boiling condition, the hydration level is maintained at a constant level as well.
When the above boiling point control is applied to a single wafer spin cleaning tool, a new set of challenges arises. In a single wafer tool, the heated etching solution is typically circulated in a circulation loop with a supply tank, a pump, a heater and a filter, and the heated etching solution that is pressurized by the pump is drawn from the circulation loop and a controlled flow of the heated etching solution is directed through a nozzle or spray bar and dispensed onto a surface of a spinning wafer. When the boiling point control described above for immersion wet benches is used in such a single wafer tool, the actual physical boiling of the heated etching solution does not occur in the heater in the circulation loop, since the pressure generated by the pump suppresses the boiling there. Instead, the boiling can occur in other places of the circulation loop where the pressure is reduced. When relatively small changes occur in either the temperature or the hydration level of the phosphoric acid in the circulation loop, the position or region where this boiling occurs can shift within the tool with detrimental effects when processing a wafer with the heated etching solution. Further, inadequate control of the temperature and hydration level of the heated etching solution can result in poor stability of the etching process, in addition to inadequate etch uniformity across large wafers.